What would happen if two neutron stars collided?

Neutron stars form when a huge star, around 8 to 30 times the mass of our Sun, explodes in a supernova. They're small, typically the size of an asteroid, around 20 kilometers in diameter, but with a mass about 1.40 times that of our Sun. To put that into perspective, a cubic centimeter of neutron star material would weigh more than ten million tonnes!

Neutron star mergers are rare, requiring systems with two massive stars that have both exploded as supernovae. According to researchers, in any one galaxy, these events occur once every million years or so. Two massive stars themselves are relatively rare compared to lower mass stars. But when that happens, the two neutron stars spiral towards one another over millions of years until they touch and coalesce and, in one of nature's most energetic explosions, unleash a short but powerful beam of gamma rays, lasting just a second or two. These short GRBs are immense bursts that emit the same amount of energy in two seconds or so, as all of the stars in our entire galaxy combined, produce in a year. Orbiting neutron stars are exactly the kind of intense gravitational environments where the ripples would be strongest.

When neutron stars collide a spectacular event ensues. In a simulation (link below), scientists placed a mismatched pair of neutron stars, weighing 1.40 and 1.70 solar masses about 20 kilometers apart, and watched the fateful event play out. As the stars start to whirl toward each other, immense tidal forces warp the crusts of the stars and the smaller star explodes, spewing its hot and dense contents that then begin spiral around the system. As the stars merge, the overwhelming mass acquired by the larger star causes it to collapse, and a black hole is born. The upper size limit for neutron stars is 3 solar masses.

When two neutron star collides, there are two possibilities: either they will merge together to form a new, larger neutron star or they will collapse into a black hole.

Case 1: Formation of larger neutron star

When the neutron star that collides weighs same solar masses, they will form a larger neutron star. Both the neutron stars have their own gravitational pull and the impact of pull will be same. Due to this they start merging together and a new, larger neutron star is formed.

Case 2: Black hole formation

When mismatched pair of neutron stars that collides weighs different solar masses, the stars start to whirl toward each other, immense tidal forces warp the crusts of the stars and the smaller star explodes (resulting due to higher difference between the gravitational pull), spewing its hot and dense contents that then begin spiral around the system. As the stars merge, the overwhelming mass acquired by the larger star causes it to collapse, and a black hole is formed.

Neutron star consist of the following:-

electrons, followed by

the nuclei of atoms (like iron), followed by

a layer where nuclei are layered (like impurities) inside an ocean of neutrons, followed by

a transition zone to the core,

where the core is a neutron superfluid (a liquid-like phase with absolutely zero friction) along with charged-particle impurities of various masses inside of it.

A strangelet is a hypothetical particle consisting of a bound state of roughly equal numbers of up, down, and strange quarks. An equivalent description is that a strangelet is a small fragment of strange matter, small enough to be considered a particle.

It is theorized that when the neutron-degenerate matter, which makes up neutron stars, is put under sufficient pressure from the star's own gravity or the initial supernova creating it, the individual neutrons break down into their constituent quarks (up quarks and down quarks), forming what is known quark matter. This conversion might be confined to the neutron star's center or it might transform the entire star, depending on the physical circumstances. Such a star is known as a quark star.

Strange matter is a particular form of quark matter, usually thought of as a "liquid" of up, down and strange quarks.

So, according to the above theory we can see that a neutron may be converted to a strangelets under some circumstances but this conversion seems to be very complicated and also doubtful and further it has not been observed till now I guess.

[Edit: The original question ("What would happen if two supernovae collided") has been changed. This answer has been left unchanged for posterity.]

Contrary to Alexandre Boyon's valiant attempt, supernovae are not stars that have gained too much energy from their environments. They are actually stars that have spent their nuclear fusion fuel (H, He). When this happens, their fusion cores no longer generate sufficient outward pressure to maintain their previous size (because fusion reactions of higher elements have a lower relative ratio of energy released to energy required), causing a collapse, and the resultant spike in density (and thus heat) causes an enormous fusion chain reaction of the remaining elements.

So, on to answer your question. Theoretically, it's possible for two nearby stars to go supernova nearly simultaneously, though probably incredibly rare compared to solo supernovae, and I don't think we've ever observed it. Technically, it's kind of hard to say that supernovae would "collide", as they have multiple components. It would be more accurate to say that their shockwaves would collide.

There are millions of potential scenarios based on dozens of variables - the relative motion and distance of each star, their planetary systems, masses and compositions, etc. However, I think it could be roughly divided into two main categories.

In the first, the stars are either motionless relative to each other, or their velocities are non-relativistic (and thus, vastly lower than that of their shockwaves). The dominant physics would then become that of wave interference and inelastic collision. You specify roughly equal powered explosions, so the shockwaves would likely cancel each other out at the point of collision and flatten along a roughly circular surface normal to the axis between the stars. This wouldn't much affect the overall shapes of the wavefronts outside this region, since the shockwaves are inelastic (i.e., it's more like pushing two grapefruit together than hitting one billiard ball with another). The cool part is that, in addition to the "normal" light show of two supernovae, all those particles colliding at relativistic speeds would make a really pretty shower of high-energy interactions. Think like the Aurora Borealis, but on crack. It'd be like seeing the inside of the LHC while it's running. Eventually, though, the whole thing would die down, and we'd get a strange-looking nebula out of the remnants. If the spent cores ever got close enough, they might merge and become a black hole, but that's about the most exciting thing that would happen.

The second scenario is not quite so elegantly poetic, but no less impressive. If the two stars are approaching each other at speeds within a factor of two (up or down) of their shockwave speeds, things look a lot different. A head-on collision would see them squish together as before, though faster, but the relativistic collision speed means it would all happen in the blink of an eye. The whole thing would explode in a chaotic mess. If the two stars were just "passing by", their shockwaves could still contact each other and technically count as a collision, but it would be more of a brief "kiss" instead of an all-night-fireworks-party (if you know what I mean). If they both had high speeds, but a low angle between their velocity vectors, we'd see the two explosions merge and spray debris all over the place like a NASCAR crash (hey, maybe that's what was up with that wierd nebula in Starfox). And finally, the last possibility is by far the coolest. If the stars approached each other at just the right velocity vectors to achieve mutual orbit, their shockwaves would weave together into a breathtaking kaleidoscope of interference waves and stellar aurorae. As the French would say, "Really fucking pretty".

Neutron stars are one of several possible endings for a star. They form when a huge star, around 8 to 30 times the mass of our Sun, explodes in a supernova (click on supernova to know more about it).

When neutron stars collide a spectacular event ensues. In this simulation, scientists placed a mismatched pair of neutron stars, weighing 1.4 and 1.7 solar masses, 11 miles apart and watched the fateful event play out. As the stars start to whirl toward each other, immense tidal forces warp the crusts of the stars and the smaller star explodes, spewing its hot and dense contents that then begin spiral around the system. As the stars merge, the overwhelming mass acquired by the larger star causes it to collapse, and a black hole is born.

These events are particularly interesting because scientists believe that they may result in short gamma-ray bursts (GRBs). These short GRBs are immense bursts that emit the same amount of energy as all of the stars in our entire galaxy combined produce in a year, in only around 2 seconds.

Actually the New York Times just reported on Monday that astronomers back in August had detected this phenomenon for the first time using the LIGO (Laser Interferometer Gravitational-Wave Observatory).

"The collision, known as a kilonova, rattled the galaxy in which it happened 130 million light-years from here in the southern constellation of Hydra, and sent fireworks across the universe. On Aug. 17, the event set off sensors in space and on Earth, as well as producing a loud chirp in antennas designed to study ripples in the cosmic fabric. "....

"Studying the fireball from this explosion, astronomers have concluded that it had created a cloud of gold dust many times more massive than the Earth, confirming kilonovas as agents of ancient cosmic alchemy."

There is a plethora of papers being published about this including one in Physical Review Letters with about 4000 authors.

Gamma Ray Bursts, the most energetic phenomena in the galaxy capable of emitting more energy than an entire galaxy, probably occur when neutron stars collide. Neutron star inspiral and collisions are also one of the most promising sources of gravitational wave sources for detection by LIGO.

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